Heterojunction bipolar transistor

ABSTRACT

A collector layer of an HBT includes a high-concentration collector layer and a low-concentration collector layer thereon. The low-concentration collector layer includes a graded collector layer in which the energy band gap varies to narrow with increasing distance from the base layer. The electron affinity of the semiconductor material for the base layer is greater than that of the semiconductor material for the graded collector layer at the point of the largest energy band gap by about 0.15 eV or less. The electron velocity in the graded collector layer peaks at a certain electric field strength. In the graded collector layer, the strength of the quasi-electric field, an electric field that acts on electrons as a result of the varying energy band gap, is between about 0.3 times and about 1.8 times the peak electric field strength, the electric field strength at which the electron velocity peaks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.16/026,841, filed Jul. 3, 2018, which is a Continuation of U.S. patentapplication Ser. No. 15/898,440 filed Feb. 17, 2018 and claims benefitof priority to Japanese Patent Application No. 2017-028690 filed Feb.20, 2017, the entire content of which is incorporated herein byreference.

BACKGROUND Technical Field

The present disclosure relates to a heterojunction bipolar transistor.

Background Art

In modern mobile terminals, heterojunction bipolar transistors (HBTs)are used as a main transistor in a high-frequency amplifier module.Examples of characteristics generally required of an HBT include highefficiency, high gain, high breakdown voltage, and high output power. Inparticular, there is a need for increased output power and efficiency inthe low-distortion region (linear region).

The maximum output power at which an HBT can operate while maintaininglinear input and output characteristics (conditions under which theadjacent channel power ratio (ACPR) is equal to or lower than areference value, such as about −40 dBc) is herein referred to as “linearoutput power,” and the efficiency of an HBT in operation at the maximumoutput power at which it can operate while maintaining linear input andoutput characteristics is referred to as “linear efficiency.” There is ademand for increased linear output power and linear efficiency.

Japanese Unexamined Patent Application Publication No. 2000-332023discloses an HBT with which high operation efficiency can be achieved.In this HBT, a collector layer is formed of AlGaAs and includes a layerin which the AlAs mixed-crystal ratio increases from 0 to 0.2 withincreasing distance from a base layer and a layer in which the AlAsmixed-crystal ratio then decreases from 0.2 to 0. At the interfacebetween the two graded layers is a two-dimensional doped layer. Thetwo-dimensional doped layer compensates for a quasi-electric fieldresulting from the difference in electron affinity and energy band gapbetween the base layer and the collector layer. Such a structure allowselectrons to pass through the base layer and collector layer withoutencountering a potential barrier.

In the HBT disclosed in Japanese Unexamined Patent ApplicationPublication No. 2000-332023, the maximum of the AlAs mixed-crystal ratioin the AlGaAs collector layer is set to 0.2. The AlAs mixed-crystalratio is set to 0.2 in order to ensure that the difference in energylevel at the upper edge of the valence band is large between the baselayer and the collector layer. This leads to advantages of the doubleheterostructure of the HBT, such as reduced offset voltage and a smallerbase-collector capacitance in the saturation region.

Japanese Unexamined Patent Application Publication No. 2006-60221discloses an HBT with improved output characteristics. This HBT has afirst collector layer, a second collector layer, and a third collectorlayer, from the subcollector layer side to the base layer side. Thedopant concentration of the first collector layer is higher than thedopant concentration of the second collector layer, and the dopantconcentration of the second collector layer is higher than the dopantconcentration of the third collector layer. Inserting the heavily dopedfirst collector layer near a subcollector layer and thereby weakeningthe electric field at the junction of the second collector layer and thesubcollector layer leads to improved on-state breakdown voltage. As aresult, an improvement in the resistance to large voltage swings, whichis essential for the betterment of output characteristics, is achieved.

In the HBT disclosed in Japanese Unexamined Patent ApplicationPublication No. 2000-332023, the dopant concentration of the collectorlayer is approximately 2×10¹⁶ cm⁻³. The in-plane dopant concentration ofthe two-dimensional dope layer, a layer disposed in the collector layer,is approximately 4.8×10¹¹ cm⁻². This heavy doping of the collector layerresults in a large base-collector voltage dependence of thebase-collector capacitance and, therefore, low linear efficiency.

In the HBT disclosed in Japanese Unexamined Patent ApplicationPublication No. 2006-60221, the dopant concentration of the thirdcollector layer, the layer closest to the base layer, is set within therange of 0.5×10¹⁶ cm⁻³ to 4×10¹⁶ cm⁻³. Setting the dopant concentrationof the third collector layer to as low as 0.5×10¹⁶ cm⁻³ would ensurelittle base-collector voltage dependence of the base-collectorcapacitance and, therefore, high linear efficiency.

However, setting the dopant concentration of the third collector layerlow would cause the electric field strength in the collector layer closeto the base layer to be small because of the Kirk effect, which becomesapparent in the high-current region (high-output-power region). As aresult, the electron velocity would slow down in the high-output-powerregion, affecting the cutoff frequency. In other words, the linearoutput power would be reduced in a region in which the operatingfrequency is high. It would therefore be difficult to give this HBT highlinear output power.

SUMMARY

Accordingly, the present disclosure provides a heterojunction bipolartransistor capable of maintaining high linear efficiency and high linearoutput power.

According to preferred embodiments of a first aspect of the presentdisclosure, a heterojunction bipolar transistor includes a substrate anda multilayer structure on the substrate, the multilayer structureincluding a collector layer, a p-type base layer, and an n-type emitterlayer. The collector layer includes a high-concentration collector layerand a low-concentration collector layer between the base layer and thehigh-concentration collector layer, the low-concentration collectorlayer having a lower dopant concentration than the high-concentrationcollector layer does. The low-concentration collector layer includes agraded collector layer in which the energy band gap varies to narrowwith increasing distance from the base layer. The electron affinity ofthe semiconductor material for the base layer is greater than theelectron affinity of the semiconductor material for the graded collectorlayer at the point of the largest energy band gap by about 0.15 eV orless. The graded collector layer is formed of a semiconductor materialin which electron velocity peaks at a certain electric field strengthwhen the electric field strength is varied. The strength of thequasi-electric field in the graded collector layer, an electric fieldthat acts on electrons as a result of the varying energy band gap, isabout 0.3 or more times and about 1.8 or less times (i.e., from about0.3 times to about 1.8 times) the peak electric field strength, theelectric field strength at which the electron velocity peaks.

The low-concentration collector layer reduces the base-collector voltagedependence of the base-collector capacity, improving the linearefficiency. A graded collector layer that meets the aboveelectron-affinity requirement mitigates the blocking effect, i.e., theinhibition of electron transport by a potential barrier existing in thecollector layer. Moreover, a graded collector layer that meets the abovequasi-electric field requirement reduces the decrease in electronvelocity that occurs in the presence of the Kirk effect. Because ofthese, the decrease in cutoff frequency that occurs in the high-currentregion is reduced, and, as a result, the loss of linear output power islimited.

According to preferred embodiments of a second aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to the first aspect, and, inaddition to this, the electron affinity of the semiconductor materialfor the base layer is greater than the electron affinity of thesemiconductor material for the graded collector layer at the point ofthe largest energy band gap by about 0.09 eV or less.

A graded collector layer that meets the above electron affinityrequirement is more effective in mitigating the blocking effect againstelectron transport. Providing such a layer therefore leads to a furtherreduction in the decrease in cutoff frequency that occurs in thehigh-current region and, as a result, more effective limitation to theloss of linear output power.

According to preferred embodiments of a third aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to the first or secondaspect, and, in addition to this, the strength of the quasi-electricfield in the graded collector layer is about 0.5 times or more and about1.3 times or less (i.e., from about 0.5 times to about 1.3 times) thepeak electric field strength.

A graded collector layer that meets the above quasi-electric fieldrequirement is more effective in reducing the decrease in electronvelocity that occurs in the presence of the Kirk effect. Providing sucha layer therefore further reduces the decrease in cutoff frequency thatoccurs in the high-current region, leading to more effective limitationto the loss of linear output power.

According to preferred embodiments of a fourth aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to any one of the first tothird aspects, and, in addition to this, the low-concentration collectorlayer is an n-type semiconductor layer with an n-type dopantconcentration of about 3×10¹⁵ cm⁻³ or less, a p-type semiconductor layerwith a p-type dopant concentration of about 1×10¹⁵ cm⁻³ or less, or anintrinsic semiconductor layer. Setting the dopant concentration of thelow-concentration collector layer as such leads to a further increase inlinear efficiency.

According to preferred embodiments of a fifth aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to any one of the first tofourth aspects, and, in addition to this, the low-concentrationcollector layer includes, besides the graded collector layer, asemiconductor layer in which the energy band gap is constant.

This ensures that even if the thickness of the low-concentrationcollector layer is optimized to increase the linear efficiency, thethickness of the graded collector layer can be set thinner than that ofthe low-concentration collector layer without being restricted by thethickness of the low-concentration collector layer. The increased degreeof freedom in the selection of the thickness of the graded collectorlayer allows the manufacturer to suitably modify the strength of thequasi-electric field in the graded collector layer. As a result, thedecrease in electron velocity that occurs in the presence of the Kirkeffect is reduced more effectively, and higher linear output power isaccomplished.

According to preferred embodiments of a sixth aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to the first to fifthaspects, and, in addition this, the base layer is formed of GaAs, thegraded collector layer is formed of AlGaAs, and the AlAs mixed-crystalratio decreases with increasing distance from the base layer.

This allows, in the fabrication of a heterojunction bipolar transistorthat meets the electron affinity, quasi-electric field, and dopantconcentration requirements, etc., according to any one of the first tofifth aspects, the manufacturer to maintain the production yield high byutilizing the mature crystal-growth technology for the AlGaAs series.Moreover, the production process does not become complicated and thecosts are not increased since a common process for producing a knownheterojunction bipolar transistor can be applied.

According to preferred embodiments of a seventh aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to the sixth aspect, and, inaddition to this, the AlAs mixed-crystal ratio in the graded collectorlayer at the interface on the base layer side is about 0.025 or more andabout 0.125 or less (i.e., from about 0.025 to about 0.125).

When the AlAs mixed-crystal ratio is set within the above range, theelectron affinity-related requirement imposed on the heterojunctionbipolar transistor according to the first aspect is met. This reducesthe decrease in electron velocity that occurs in the presence of theKirk effect, and brings down the decrease in cutoff frequency thatoccurs in the high-current region. As a result, the loss of linearoutput power is limited.

According to preferred embodiments of an eighth aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to any one of the first toseventh aspects, and, in addition to this, further has an n-typesubcollector layer on the substrate. The collector layer, base layer,and emitter layer are stacked in this order on part of the surfaceregion of the subcollector layer, and the high-concentration collectorlayer includes a first layer on the subcollector layer side and a secondlayer on the low-concentration collector layer side. The dopantconcentration of the first layer and the dopant concentration of thesubcollector layer are higher than the dopant concentration of thesecond layer.

The first layer is inserted to be parallel with the subcollector layerin the path of collector current. This reduces the collector resistance,making progress in increasing the output power and improving theefficiency of power amplifiers equipped with a heterojunction bipolartransistor.

According to preferred embodiments of a ninth aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to the eighth aspect, and,in addition to this, the dopant concentration of the second layer andthe dopant concentration of the low-concentration collector layer areequal to or less than about 1/10 of the dopant concentration of thefirst layer.

This prevents a decrease in base-collector breakdown voltage andcollector-emitter breakdown voltage, thereby preventing the breakdown ofthe heterojunction bipolar transistor that would otherwise occur whenthe output voltage swings to a maximum extent at full high-frequencyoutput power.

According to preferred embodiments of a tenth aspect of the presentdisclosure, a heterojunction bipolar transistor has the structure of aheterojunction bipolar transistor according to any one of the first toninth aspects, and, in addition to this, the low-concentration collectorlayer includes a reversely graded collector layer between the gradedcollector layer and the base layer, the energy band gap of the reverselygraded collector layer varies in the thickness direction, the energyband gap of the reversely graded collector layer at the interface on thebase layer side is equal to the energy band gap of the base layer, and,at the interface between the graded collector layer and the reverselygraded collector layer, the energy band gap of the graded collectorlayer and the energy band gap of the reversely graded collector layerare equal.

Inserting such a reversely graded collector layer between the gradedcollector layer and the base layer lowers the potential barrier thatacts on the electrons that flow from the base layer into the collectorlayer in the presence of emitter-collector voltage. This reduces thedecrease in the velocity of electrons transported in the collector,limiting the loss of linear output power.

Other features, elements, characteristics and advantages of the presentdisclosure will become more apparent from the following detaileddescription of preferred embodiments of the present disclosure withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross-sectional view of an HBT according toEmbodiment 1, and FIG. 1B is a schematic cross-sectional diagram thatincludes a graph that shows an example of a change in mixed-crystalratio in a graded collector layer;

FIG. 2 is a graph that shows a simulated relationship between themagnitude of collector current and cutoff frequency for various maximummixed-crystal ratios x_(MAX) in the graded collector layer (FIG. 1A) ofan HBT according to Embodiment 1;

FIG. 3 is a graph that shows a simulated distribution of electrondensities in the base layer and collector layer (FIG. 1A);

FIG. 4 is a graph that shows simulated effective electric fields in alow-concentration collector layer;

FIGS. 5A and 5B are graphs that show the relationship between electricfield strength and electron velocity in an AlGaAs layer;

FIG. 6 is a graph that shows a simulated relationship between maximummixed-crystal ratio x_(MAX) and the electron velocity at the point ofthe largest AlAs mixed-crystal ratio for different thicknesses of agraded collector layer;

FIG. 7 is a schematic cross-sectional view of an HBT according toEmbodiment 2;

FIG. 8A is a schematic cross-sectional view of an HBT according toEmbodiment 2 in production;

FIG. 8B is a schematic cross-sectional view of an HBT according toEmbodiment 2 in production;

FIG. 8C is a schematic cross-sectional view of an HBT according toEmbodiment 2 in production;

FIG. 8D is a schematic cross-sectional view of a finished HBT accordingto Embodiment 2;

FIG. 9 is a schematic cross-sectional view of an HBT according to avariation of an HBT according to Embodiment 2, a variation in which noreversely graded collector layer is provided;

FIG. 10A is a schematic cross-sectional view of an HBT according toReference Configuration 1, and FIG. 10B is a schematic cross-sectionalview of an HBT according to Reference Configuration 2; and

FIG. 11 is a graph that shows a simulated relationship between thecutoff frequency and collector current of an HBT.

DETAILED DESCRIPTION

Before the description of embodiments, the high-frequencycharacteristics of HBTs according to Reference Configurations 1 and 2,which have a structure similar to that of the HBTs presented in thecitation, are described with reference to FIGS. 10A, 10B, and 11. Toevaluate the high-frequency characteristics, the inventors determinedthe relationship of cutoff frequency, which provides a measure ofhigh-frequency characteristics, and collector current by simulation.

FIG. 10A is a schematic cross-sectional view of an HBT according toReference Configuration 1. There is an n-type GaAs subcollector layer101 on a semi-insulating GaAs substrate 100. An n-type GaAs collectorlayer 102 and a p-type GaAs base layer 110 are stacked in this order onpart of the surface region of the subcollector layer 101. An n-typeInGaP emitter layer 111, a cap layer 112, and a contact layer 113 arestacked in this order on part of the surface region of the base layer110.

A collector electrode 115 is ohmically coupled to the subcollector layer101. A base electrode 116 is ohmically coupled to the base layer 110. Anemitter electrode 117 is ohmically coupled to the emitter layer 111 withthe contact layer 113 and cap layer 112 interposed therebetween.

The collector layer 102 has a multilayer structure in which, in orderfrom the substrate 100 side, a first collector layer 103, a secondcollector layer 104, a third collector layer 105, and a fourth collectorlayer 106 are stacked. The first collector layer 103 has a dopantconcentration of about 5×10¹⁸ cm⁻³ and a thickness of about 325 nm. Thesecond collector layer 104 has a dopant concentration of about 4.6×10¹⁶cm⁻³ and a thickness of about 190 nm. The third collector layer 105 hasa dopant concentration of about 1.5×10¹⁶ cm⁻³ and a thickness of about210 nm. The fourth collector layer 106 has a dopant concentration ofabout 3×10¹⁵ cm⁻³ and a thickness of about 400 nm.

In the HBT according to Reference Configuration 1, the dopantconcentration of the collector layer 102 is relatively low on the baselayer 110 side and relatively high on the subcollector layer 101 side.This distribution of dopant concentrations is similar to thedistribution of dopant concentrations in the collector layers of the HBTdisclosed in Japanese Unexamined Patent Application Publication No.2006-60221.

FIG. 10B is a schematic cross-sectional view of an HBT according toReference Configuration 2. In an HBT according to ReferenceConfiguration 2, the fourth collector layer 106 in an HBT according toReference Configuration 1 is composed of a graded collector layer 106A,on the lower side, and a reversely graded collector layer 106B, on theupper side. The dopant concentrations of the graded collector layer106A, on the lower side, and the reversely graded collector layer 106B,on the upper side, are both about 1×10¹⁴ cm⁻³, set to be lower than thedopant concentration of the fourth collector layer 106 in ReferenceConfiguration 1 for higher linear efficiency. The rest of the structureof the HBT according to Reference Configuration 2 is the same as in thestructure of the HBT according to Reference Configuration 1.

The graded collector layer 106A and reversely graded collector layer106B are formed of n-type AlGaAs. The AlAs mixed-crystal ratio in AlGaAsis represented by x. The AlAs mixed-crystal ratio x refers to therelative number of moles of AlAs in a mixed crystal composed of GaAs andAlAs. For example, an AlGaAs mixed crystal with an AlAs mixed-crystalratio of x is described as Al_(x)Ga_(1-x)As. The AlAs mixed-crystalratio x in the reversely graded collector layer 106B is about 0 at theinterface with the base layer 110, and about 0.2 at the interface withthe graded collector layer 106A, on the lower side. The AlAsmixed-crystal ratio x in the graded collector layer 106A is about 0.2 atthe interface with the reversely graded collector layer 106B, on theupper side, and about 0 at the interface with the third collector layer105. Inside the graded collector layer 106A and reversely gradedcollector layer 106B, the AlAs mixed-crystal ratio x varies linearly inthe thickness direction.

The HBT according to Reference Configuration 2 is structurally similarto the HBT disclosed in Japanese Unexamined Patent ApplicationPublication No. 2000-332023 in that it has a graded collector layer 106Aand a reversely graded collector layer 106B in which the AlAsmixed-crystal ratio x varies in the thickness direction. Moreover, thedistribution of dopant concentrations in the collector layer 102 of theHBT according to Reference Configuration 2 is similar to thedistribution of dopant concentrations in the collector layers of the HBTdisclosed in Japanese Unexamined Patent Application Publication No.2006-60221 in that the dopant concentration is relatively low on thebase layer 110 side and relatively high on the subcollector layer 101side. That is, the HBT according to Reference Configuration 2 has bothof a feature of the HBT disclosed in Japanese Unexamined PatentApplication Publication No. 2000-332023 and a feature of the HBTdisclosed in Japanese Unexamined Patent Application Publication No.2006-60221.

For an HBT according to Reference Configuration 1 (FIG. 10A) and an HBTaccording to Reference Configuration 2 (FIG. 10B), the relationshipbetween cutoff frequency and collector current was determined bysimulation. The following describes the results of the simulations.

FIG. 11 is a graph that shows a simulated relationship between cutofffrequency and collector current. The horizontal axis represents themagnitude of collector current, and the vertical axis represents cutofffrequency. The solid line 10A in the graph indicates the cutofffrequency of the HBT according to Reference Configuration 1 (FIG. 10A),and the broken line 10B indicates the cutoff frequency of the HBTaccording to Reference Configuration 2 (FIG. 10B). Note that FIG. 11shows the cutoff frequencies in and near the saturation region, whichplays an important role in increasing output power, of the HBTs. Thesimulations were performed under conditions such that the base-emittervoltage and the collector-emitter voltage would be equal.

It can be seen that in Reference Configuration 1, as shown with thesolid line 10A, the cutoff frequency decreases with increasing collectorcurrent in the high-current region, which is necessary for increasingoutput power. The following discusses the cause of the decrease incutoff frequency.

As the collector current increases, the charge of electrons running inthe collector layer 102 (FIG. 10A) becomes not negligible for the spacecharge, for example of donors, changing the shape of the potential ofthe collector. The electric field of the region in the collector layer102 closer to the base layer 110 decreases, slowing down the electronvelocity. The resulting increase in base-collector diffusion capacitanceand the subsequent increase in the transit time of electrons passingthrough the region in the collector layer 102 closer to the base layer110 affect the cutoff frequency. Such an effect is referred to as Kirkeffect.

When the dopant concentration is set low on the base layer 110 side ofthe collector layer 102 as in Reference Configuration 1, it is difficultto maintain the cutoff frequency high in the high-current region becauseof the influence of the Kirk effect.

It can be seen that in Reference Configuration 2, as shown with thebroken line 10B, the cutoff frequency markedly decreases in thehigh-current region. The following discusses the cause of the decreasein cutoff frequency.

In Reference Configuration 2, the electron affinity of the AlGaAsreversely graded collector layer 106B and graded collector layer 106A(FIG. 10B) is smaller than the electron affinity of the GaAs base layer110 (FIG. 10B). The energy level of electrons at the lower edge of theconduction band of the graded collector layer 106A and reversely gradedcollector layer 106B is therefore higher than the energy level ofelectrons at the lower edge of the conduction band of the base layer110. In particular, the interface between the reversely graded collectorlayer 106B and the graded collector layer 106A works as a potentialbarrier against electrons transported in the collector layer 102 becausethe AlAs mixed-crystal ratio is the highest at this interface. Thiseffect whereby a potential barrier inhibits electron transport isreferred to as blocking effect.

In Reference Configuration 2, the blocking effect is significant becauseof low dopant concentrations of the graded collector layer 106A andreversely graded collector layer 106B. In particular, in thehigh-current region, the charge of electrons running in the collectorlayer 102 (FIG. 10A) is not negligible for the space charge, for exampleof donors, and the blocking effect is more significant. As a result, thecutoff frequency is markedly low in the high-current region. It istherefore difficult to accomplish high linear output power.

To mitigate the blocking effect, the manufacturer can set the dopantconcentration high on the base layer side of the collector layer orprovide a two-dimensional doped layer as in the HBT disclosed inJapanese Unexamined Patent Application Publication No. 2000-332023. Insuch a configuration, however, the linear efficiency is low because oflarge base-collector voltage dependence of the base-collectorcapacitance.

As seen from the foregoing, it is difficult to achieve high linearefficiency and high linear output power at the same time with theexisting approach of reducing concentrations in the collector layer orintroducing a graded collector layer. In contrast, the embodimentsdescribed below accomplish both of high linear efficiency and highlinear output power.

Embodiment 1

The following describes an HBT according to Embodiment 1 with referenceto FIG. 1A to FIG. 6.

FIG. 1A is a schematic cross-sectional view of an HBT according toEmbodiment 1. There is an n-type GaAs subcollector layer 21 on a GaAssubstrate 20. A collector layer 30 and a p-type GaAs base layer 40 arestacked in this order on part of the surface region of the subcollectorlayer 21.

The collector layer 30 includes a high-concentration collector layer 31and a low-concentration collector layer 32. The low-concentrationcollector layer 32 is disposed between the high-concentration collectorlayer 31 and the base layer 40. The dopant concentration of thelow-concentration collector layer 32 is lower than the dopantconcentration of the high-concentration collector layer 31. FIG. 1Aillustrates an example in which the high-concentration collector layer31 is in contact with the subcollector layer 21, and thelow-concentration collector layer 32 is in contact with the base layer40. It should be understood that the high-concentration collector layer31 does not need to be in direct contact with the subcollector layer 21,the low-concentration collector layer 32 does not need to be in directcontact with the base layer 40, and there may be another layer betweenthe low-concentration collector layer 32 and the base layer 40.

Part or the entirety in the thickness direction of the low-concentrationcollector layer 32 is a graded collector layer 34 made of n-type AlGaAs.The graded collector layer 34 is formed of a mixed-crystal semiconductordifferent from the semiconductors that form the subcollector layer 21and the base layer 40, and has mixed-crystal ratios varying (graded) inthe thickness direction. The mixed-crystal ratio varies so that theenergy band gap tapers from the base layer 40 side to subcollector layer21 side. On the substrate 20 side of the low-concentration collectorlayer 32 with respect to the graded collector layer 34, themixed-crystal ratio is constant in the thickness direction.

FIG. 1B is a schematic cross-sectional diagram that includes a graphthat shows an example of a change in the mixed-crystal ratio in thegraded collector layer 34. The graded collector layer 34 is formed ofn-type Al_(x)Ga_(1-x)As, and the AlAs mixed-crystal ratio x in thegraded collector layer 34 tapers from the base layer 40 side to thesubcollector layer 21 side. The AlAs mixed-crystal ratio x at theinterface on the subcollector layer 21 side is about 0. That is, at theinterface of the graded collector layer 34 closest to the subcollectorlayer 21, the energy band gap of the graded collector layer 34 is equalto the energy band gap of the subcollector layer 21. The AlAsmixed-crystal ratio x at the interface on the base layer 40 side isrepresented by x_(MAX). This mixed-crystal ratio x_(MAX) is hereinafterreferred to as maximum mixed-crystal ratio. The AlAs mixed-crystal ratiox varies linearly in the graded collector layer 34 in the thicknessdirection.

As illustrated in FIG. 1A, an n-type InGaP emitter layer 41, a cap layer42, and a contact layer 43 are stacked in this order on part of thesurface region of the base layer 40. A collector electrode 45 makesohmic contact with the subcollector layer 21. A base electrode 46 makesohmic contact with the base layer 40. An emitter electrode 47 makesohmic contact with the contact layer 43, ohmically coupling the emitterelectrode 47 to the emitter layer 41.

The following describes how the cutoff frequency varies with maximummixed-crystal ratio x_(MAX) in the high-current region, with referenceto FIG. 2.

FIG. 2 is a graph that shows a simulated relationship between themagnitude of collector current and cutoff frequency for multiple samplesof HBTs according to Embodiment 1 with different maximum mixed-crystalratios x_(MAX) of the graded collector layer 34 (FIG. 1A). Note thatalthough FIG. 2 shows the results of simulations performed on sampleshaving the structure illustrated in FIG. 10B with different maximummixed-crystal ratios x_(MAX), the structure in FIG. 10B can be deemed tobe substantially identical to the structure in FIG. 1A. For example, thegraded collector layer 34 in FIG. 1A corresponds to the graded collectorlayer 106A in FIG. 10B. The high-concentration collector layer 31 inFIG. 1A corresponds to the three layers of the first collector layer103, second collector layer 104, and third collector layer 105 in FIG.10B. The layer between the graded collector layer 34 and the base layer40 in FIG. 1A corresponds to the reversely graded collector layer 106Bin FIG. 10B. Simulations were performed on multiple maximummixed-crystal ratios x_(MAX) between about 0.0 and about 0.2. FIG. 2shows the behavior of cutoff-frequency determined in and near thesaturation region, which plays an important role in increasing outputpower, of the HBTs. Maximum mixed-crystal ratios x_(MAX) are givencorresponding one-to-one to the solid or broken lines in the graph inFIG. 2. Note that the simulations were performed under conditions suchthat the emitter-base voltage Vbe would be equal to theemitter-collector voltage Vce.

In the high-current region, at maximum mixed-crystal ratios x_(MAX) ofabout 0 to about 0.075, the cutoff frequency becomes higher withincreasing maximum mixed-crystal ratio x_(MAX). At maximum mixed-crystalratios x_(MAX) of about 0.075 or more and about 0.1 or less (i.e., fromabout 0.075 to about 0.1), the cutoff frequency decreases withincreasing maximum mixed-crystal ratio x_(MAX), but the decrease incutoff frequency is small. At maximum mixed-crystal ratios x_(MAX)exceeding about 0.1, the decrease in cutoff frequency associated with anincrease in maximum mixed-crystal ratio x_(MAX) is large, and at maximummixed-crystal ratios x_(MAX) exceeding about 0.125, the decrease incutoff frequency associated with an increase in maximum mixed-crystalratio x_(MAX) is markedly large. From the simulation results shown inFIG. 2, it can be seen that there is a maximum mixed-crystal ratiox_(MAX) best for reducing the decrease in cutoff frequency (foraccomplishing high linear output power) in the high-current region. Inthe example illustrated in FIG. 2, the best maximum mixed-crystal ratiox_(MAX) is in the vicinity of about 0.075.

The following describes the reason why the relationship betweencollector current and cutoff frequency varies with maximum mixed-crystalratio x_(MAX), separately for the case in which the maximummixed-crystal ratio x_(MAX) is about 0.1 or more and the case in whichx_(MAX) is about 0.1 or less.

Maximum Mixed-Crystal Ratio x_(MAX) is about 0.1 or More

At maximum mixed-crystal ratios x_(MAX) of about 0.1 or more, as shownin FIG. 2, the cutoff frequency decreases with increasing maximummixed-crystal ratio x_(MAX). In particular, at maximum mixed-crystalratios x_(MAX) of about 0.15 or more, not only the cutoff frequencydecreases with increasing maximum mixed-crystal ratio x_(MAX) in thehigh-current region, but also the peak maximum mixed-crystal ratiox_(MAX) decreases in the low-current region. The decrease in cutofffrequency and the decrease in peak maximum mixed-crystal ratio x_(MAX)result from, as described with reference to FIG. 11, the blocking effectcaused by a potential barrier in the collector layer acting on electrontransport becoming apparent.

The following describes the grounds for the belief that the blockingeffect is apparent at maximum mixed-crystal ratios x_(MAX) of about 0.1or more, with reference to FIG. 3.

FIG. 3 is a graph that shows a simulated distribution of electrondensities in the base layer 40 and collector layer 30 (FIG. 1A). Thehorizontal axis represents the position in the thickness direction inthe base layer 40 and collector layer 30, with one space correspondingto 0.1 μm. The vertical axis represents electron density in the unit of“cm⁻³.” Simulations were performed on multiple maximum mixed-crystalratios x_(MAX) in the range of about 0.0 to about 0.2. Maximummixed-crystal ratios x_(MAX) are given corresponding one-to-one to thesolid or broken lines in the graph in FIG. 3. Note that the simulationresults shown in FIG. 3 represent electron densities in a state in whicha collector current in the high-current region, specifically thecollector current Ic0 indicated in FIG. 2, is flowing.

When the maximum mixed-crystal ratio x_(MAX) is about 0.15 or about 0.2,the electron density sharply peaks near the interface between the baselayer 40 and the collector layer 30. This indicates that electrontransport is being blocked by a potential barrier formed at the point ofthe largest energy band gap in the graded collector layer 34 (FIG. 1B).The blocked electron transport from the base layer 40 to the collectorlayer 30 leads to the accumulation of electrons and therefore increasedelectron density in the base layer 40, resulting in an increase inemitter diffusion capacitance Cbe. The increase in emitter diffusioncapacitance Cbe leads to a decrease in cutoff frequency as shown in FIG.2.

When the maximum mixed-crystal ratio x_(MAX) is about 0.0, no blockingeffect occurs since no potential barrier is formed against electrons.The electron density in the base layer 40 at a maximum mixed-crystalratio x_(MAX) of about 0.05 is substantially equal to the electrondensity at a maximum mixed-crystal ratio x_(MAX) of about 0.0. Thismeans that when the maximum mixed-crystal ratio x_(MAX) is about 0.05,the blocking effect is substantially absent, and therefore the decreasein cutoff frequency is limited as shown in FIG. 2.

The increase in electron density in the base layer 40 is slight evenwhen the maximum mixed-crystal ratio x_(MAX) is increased to about 0.1,indicating controlled influence of the blocking effect. The differencebetween the electron affinity of an AlGaAs with an AlAs mixed-crystalratio of about 0.1 and the electron affinity of GaAs is about 0.12 eV.It is therefore preferred to select the semiconductor material for thebase layer 40 and the semiconductor material for and the maximummixed-crystal ratio x_(MAX) in the graded collector layer 34 to meet thefirst and second requirements below so that the influence of theblocking effect will be controlled.

The first requirement can be that the electron affinity of thesemiconductor material that forms the base layer 40 be greater than theelectron affinity of a semiconductor material that has the compositionthat the graded collector layer 34 has in the peak portion, the portionin which the lower edge of the conduction band of the layer is thehighest (the point at which the energy band gap is the largest). Thesecond requirement can be that the difference in electron affinitybetween the two semiconductor materials be about 0.12 eV or less.

Moreover, as can be seen from FIG. 2, the cutoff frequency is thehighest in the high-current region when the maximum mixed-crystal ratiox_(MAX) is about 0.075. The difference between the electron affinity ofan AlGaAs with an AlAs mixed-crystal ratio x of about 0.075 and theelectron affinity of GaAs is about 0.09 eV. It is therefore morepreferred to replace the second requirement with the requirement thatthe difference in electron affinity between the two semiconductormaterials be about 0.09 eV or less.

Furthermore, as can be seen from FIG. 2, the cutoff frequency is notmarkedly low in the high-current region when the maximum mixed-crystalratio x_(MAX) is about 0.125, either. The difference between theelectron affinity of an AlGaAs with an AlAs mixed-crystal ratio x ofabout 0.125 and the electron affinity of GaAs is about 0.15 eV. Thesecond requirement may therefore be replaced with the requirement thatthe difference in electron affinity between the two semiconductormaterials be about 0.15 eV or less.

Maximum Mixed-Crystal Ratios x_(MAX) is about 0 or More and about 0.1 orLess (i.e., from about 0 to about 0.1)

The electric field in the fourth collector layer 106 (FIG. 10A), alow-concentration collector layer, becomes small in the high-currentregion because of the Kirk effect. In the fourth collector layer 106,therefore, whereas diffusion current contributes greatly to thecollector current, drift current contributes little. Electronsaccumulate in the fourth collector layer 106 to such a density that thestream of electrons flowing from the base layer 110 (FIG. 10A) into thefourth collector layer 106 is maintained, increasing the base-collectordiffusion capacitance and the transit time of electrons in the collectorlayer. As a result, as shown in FIG. 2 with the x_(MAX)=0.0 solid line,the cutoff frequency decreases with increasing collector current in theregion in which the current is higher than the collector current atwhich the cutoff frequency peaks. As is clear from this, the decrease incutoff frequency with increasing collector current in the high-currentregion is a result of the Kirk effect.

The inventors for the present application found that even in thehigh-current region, in which the Kirk effect occurs, the effectiveelectric field in the low-concentration collector layer 32 can becontrolled by adjusting the energy band gap of the graded collectorlayer 34 (FIGS. 1A and 1B). The following demonstrates, on the basis ofsimulation results, that the effective electric field in thelow-concentration collector layer 32 can be controlled, with referenceto FIG. 4.

FIG. 4 is a graph that shows simulated effective electric fields in thelow-concentration collector layer 32. The horizontal axis represents theposition in the thickness direction, with one space corresponding to 0.1μm. The vertical axis represents effective electric field in the unit of“V/cm.” An effective electric field directed from the emitter layer 41to the collector layer 30 (FIG. 1A) was defined as positive. Thesimulation results shown in FIG. 4 represent effective electric fieldsin a state in which a collector current in the high-current region,specifically the collector current Ic0 indicated in FIG. 2, is flowing.

The structures of the simulated HBTs are identical to those illustratedin FIGS. 10A and 10B. The broken line in the graph in FIG. 4 indicateseffective electric fields simulated for an HBT illustrated in FIG. 10A(maximum mixed-crystal ratio x_(MAX)=about 0.0), and the solid lineindicates effective electric fields simulated for an HBT illustrated inFIG. 10B (maximum mixed-crystal ratio x_(MAX)=about 0.05). The region inwhich the effective electric field is positive corresponds to thereversely graded collector layer 106B illustrated in FIG. 10B.

The effective electric field can be separated into a component resultingfrom electrostatic potential (hereinafter referred to as “externalelectric field”) and a component resulting from the tapering energy bandgap of the graded collector layer 106A (FIG. 10B) (hereinafter referredto as “quasi-electric field”). The quasi-electric field can be furtherseparated into a component due to a gradient in the energy level of thelower edge of the conduction band and a component due to a gradient inthe effective density of states in the conduction band.

In the HBT with x_(MAX)=about 0.0 in FIG. 4, there is no quasi-electricfield. Therefore, the effective electric field is totally the externalelectric field. In the HBT with x_(MAX)=about 0.05, which includes thegraded collector layer 106A, the effective electric field is equal tothe sum of the external electric field and the quasi-electric field. Itcan be seen that providing the graded collector layer 106A makes theabsolute effective electric field great as compared with that in the HBTin FIG. 10A, which has no graded collector layer. The difference ineffective electric field between the two HBTs is roughly between 1000V/cm and 1400 V/cm. It can also be seen that in the region in which theabsolute effective electric field is the smallest, giving a gradient tothe energy band gap of the low-concentration collector layer 32 leads toan about twofold increase in absolute effective electric field.

In the region in which the current is low enough that the Kirk effect isnot apparent, the external electric field is predominant in the gradedcollector layer 106A, and the quasi-electric field is negligibly smallcompared with the external electric field. In the high-current region,the quasi-electric field is unignorably large in relation to theexternal electric field because the effective electric field in thegraded collector layer 106A is small as a result of the Kirk effectbeing apparent. The advantage of providing the graded collector layer106A is particularly great in the high-current region, in which the Kirkeffect is apparent.

In the foregoing, it has been shown that providing a graded collectorlayer 106A increases the absolute effective electric field. Thisincrease in effective electric field varies as the maximum mixed-crystalratio x_(MAX) in the graded collector layer 106A is adjusted. Thefollowing describes a preferred range of effective electric fields forincreasing the linear output power by reducing the decrease in cutofffrequency and a preferred range of maximum mixed-crystal ratios x_(MAX),with reference to FIGS. 5A and 5B.

FIGS. 5A and 5B are graphs that show the relationship between electricfield strength and electron velocity in an AlGaAs layer. These graphshave been published in Journal of Applied Physics, vol. 87, p. 2890(2000) by S. Hava and M. Auslender. The horizontal axis in FIGS. 5A and5B represents electric field strength in the unit of “kV/cm,” and thevertical axis represents electron velocity in the unit of “×10⁷ cm/s.”The numeric values paired with the curves in the graphs in FIGS. 5A and5B indicate AlAs mixed-crystal ratios.

It can be seen that AlGaAs is a material in which the electron velocitypeaks at a certain electric field strength when the electric fieldstrength is varied. Within the range of AlAs mixed-crystal ratios ofabout 0.0 or more and about 0.1 or less (i.e., from about 0.0 to about0.1), the electron velocity is the maximum at an electric field strengthof approximately 3400 V/cm. The electric field strength at which theelectron velocity peaks is hereinafter referred to as “peak electricfield strength.”

Operating the HBT under conditions such that the effective electricfield in the low-concentration collector layer 32 (FIG. 1A) comes nearthe peak electric field strength accelerates electrons in thelow-concentration collector layer 32, reducing the decrease in cutofffrequency that occurs in the high-current region. When the strength ofthe effective electric field falls within the range of about 0.5 or moretimes to about 2 or less times (i.e., from about 0.5 times to about 2times) the peak electric field strength, an electron velocity equal toor higher than about 70% of the peak value is achieved. When thestrength of the effective electric field falls within the range of about0.7 or more times to about 1.5 or less times (i.e., from about 0.7 timesto about 1.5 times) the peak electric field strength, an electronvelocity equal to or higher than about 90% of the peak value isachieved. As is clear from this, setting the strength of the effectiveelectric field within the above ranges will ensure that the electronvelocity remains high, resulting in a smaller decrease in cutofffrequency.

The effective electric field, as already described, is defined as thesum of the external electric field and the quasi-electric field. It wasdetermined by simulation that at collector-emitter voltages in and nearthe saturation region, which plays an important role for increasingoutput power, of an HBT, the contribution of the external electric fieldto the effective electric field is approximately 700 V/cm. The preferredrange of quasi-electric fields for maintaining the electron velocityhigh can be determined by subtracting the strength of the externalelectric field from the strength of the effective electric field.Subtracting the strength of the external electric field from thestrength of the effective electric field gives the result that to makethe electron velocity equal to or higher than about 70% of the peakvalue, it is recommended to set the strength of the quasi-electric fieldabout 0.3 or more times and about 1.8 or less times (i.e., from about0.3 times to about 1.8 times) the peak electric field strength. To makethe electron velocity equal to or higher than about 90% of the peakvalue, it is recommended to set the strength of the quasi-electric fieldabout 0.5 or more times and about 1.3 or less times (i.e., from about0.5 times to about 1.3 times).

The following describes the relationship between the maximummixed-crystal ratio x_(MAX) in the graded collector layer 106A (FIG.10B) and preferred thicknesses of the graded collector layer 106A withreference to FIG. 6. Changing the maximum AlAs mixed-crystal ratiox_(MAX) not only results in a change in the magnitude of thequasi-electric field but also, as shown in FIGS. 5A and 5B, leads to achange in the electron velocity with the AlAs mixed-crystal ratio x inthe graded collector layer 106A. To simplify the calculations of theelectron velocity in the graded collector layer 106A, it was assumedthat the AlAs mixed-crystal ratio x in the graded collector layer 106Awould vary linearly.

FIG. 6 is a graph that shows the relationship between maximummixed-crystal ratio x_(MAX) and the electron velocity at the point ofthe largest AlAs mixed-crystal ratio as determined for differentthicknesses of the graded collector layer 106A through calculationsbased on the graphs in FIGS. 5A and 5B. The horizontal axis representsmaximum mixed-crystal ratio x_(MAX), and the vertical axis representselectron velocity in the unit of “×10⁷ cm/s.” In the graph in FIG. 6,the thinnest solid line, second-thinnest solid line, and thickest solidline indicate the results of calculations in which the thickness of thegraded collector layer 106A was about 200 nm, about 400 nm, and about600 nm, respectively.

As shown in FIG. 6, the electron velocity reaches a maximum at a certainmaximum mixed-crystal ratio x_(MAX) regardless of the thickness of thegraded collector layer 106A. It can be seen that the cutoff frequencycan be maximized by selecting the maximum mixed-crystal ratio x_(MAX) atwhich the electron velocity reaches a maximum. This helps understand thetrend shown in FIG. 2, in which within the range of maximummixed-crystal ratios x_(MAX) of about 0.075 or less, the cutofffrequency increases with increasing maximum mixed-crystal ratio x_(MAX)and peaks at a maximum mixed-crystal ratio x_(MAX) of about 0.075 in thehigh-current region.

As stated, the fastest electron velocity can be used by selecting thebest maximum mixed-crystal ratio x_(MAX) according to the thickness ofthe graded collector layer 34 (FIGS. 1A and 1B) within the range ofmaximum mixed-crystal ratios x_(MAX) of about 0.025 or more and about0.125 or less (i.e., from about 0.025 to about 0.125). Maximizing theelectron velocity maximizes the cutoff frequency in the high-currentregion.

Moreover, from FIG. 6, it can be seen that the maximum electron velocityincreases with decreasing thickness of the graded collector layer 34.This means that thinning the graded collector layer 34 increases themaximum electron velocity achievable in the presence of the Kirk effect.Thinning the graded collector layer 34 therefore leads to a smallerdecrease in cutoff frequency in the high-current region. As a result,such an HBT offers a smaller loss of linear output power as well as theimproved linear efficiency owing to the low-concentration collectorlayer 32.

Advantages of Embodiment 1

The above description of Embodiment 1 gives the following findings.

Making a region of the collector layer 30 (FIGS. 1A and 1B) on the baselayer 40 side (corresponding to the low-concentration collector layer32) a low-concentration region reduces the base-collector voltagedependence of the base-collector capacitance in the same way as inReference Configuration 1, illustrated in FIG. 10A. This leads to anincrease in linear efficiency.

Making a region of the collector layer 30 on the base layer 40 side alow-concentration region, however, affects the linear output power bycausing the Kirk effect to be apparent in the high-current region. InEmbodiment 1, there is in a low-concentration collector layer 32 agraded collector layer 34 in which the energy band gap narrows in thedirection of increasing distance from the base layer 40, and this limitsthe loss of linear output power resulting from the Kirk effect. Inaddition to this, setting the strength of the effective electric fieldwithin a range that includes the peak electric field strength willensure that the electron velocity remains high in the low-concentrationcollector layer 32, resulting in a further improvement in cutofffrequency.

Making the electron affinity of the semiconductor material for thegraded collector layer 34 at the point of the largest energy band gapsmaller than the electron affinity of the semiconductor material for thebase layer 40 and making the difference between the two electronaffinities about 0.15 eV or less advantageously limits the loss oflinear output power. Making the difference between the two electronaffinities about 0.12 eV or less is highly effective in limiting theloss of linear output power, and making the difference about 0.09 eV orless is more effective.

Variation of Embodiment 1

The following describes a variation of Embodiment 1. In Embodiment 1, asillustrated in FIGS. 1A and 1B, the graded collector layer 34 is insidethe low-concentration collector layer 32 in the thickness direction.That is, there are layers of the low-concentration collector layer 32other than the graded collector layer 34 between the graded collectorlayer 34 and the base layer 40 and between the graded collector layer 34and the high-concentration collector layer 31. Alternatively, the gradedcollector layer 34 may be in direct contact with one of the base layer40 and the high-concentration collector layer 31 or both.

Although in Embodiment 1 the graded collector layer 34 is an AlGaAslayer, other mixed-crystal semiconductor materials may also be used. Forexample, the graded collector layer 34 can be a layer of GaInNAs,InGaAs, or GaAsSb, to name but a few.

Embodiment 2

The following describes an HBT according to Embodiment 2 with referenceto FIG. 7 to FIG. 8D. In the following, the elements this embodiment hasin common with Embodiment 1 may be mentioned without description.

FIG. 7 is a schematic cross-sectional view of an HBT according toEmbodiment 2. The individual elements of the HBT illustrated in FIG. 7are given the same reference numerals as the reference numerals given tothe corresponding elements of the HBT according to Embodiment 1illustrated in FIG. 1A.

The high-concentration collector layer 31 includes, in order from thesubstrate 20 side, a lower collector layer 31 a, a middle collectorlayer 31 b, and an upper collector layer 31 c. The graded collectorlayer 34 of the low-concentration collector layer 32 is in directcontact with the high-concentration collector layer 31. Thelow-concentration collector layer 32 further includes a reversely gradedcollector layer 35 between the graded collector layer 34 and the baselayer 40. Whereas the energy band gap of the graded collector layer 34narrows with increasing distance from the base layer 40, the energy bandgap of the reversely graded collector layer 35 broadens with increasingdistance from the base layer 40.

There is an emitter layer 41 on the entire surface of the base layer 40.It should be noted that the emitter layer 41 is depleted in the region41 d, or except immediately beneath the cap layer 42. The base electrode46 is inside openings created in the emitter layer 41, making ohmiccontact with the base layer 40.

The contact layer 43 includes a lower contact layer 43 a and an uppercontact layer 43 b thereon. The lower contact layer 43 a has varyingmixed-crystal ratios in the thickness direction and functions to relaxdistortion.

The following gives an example of the material for, dopant concentrationof, and thickness of each layer. The substrate 20 is a semi-insulatingGaAs substrate.

The subcollector layer 21 is formed of n-type GaAs, its silicon (Si)concentration is about 2×10¹⁸ cm⁻³ or more and about 6×10¹⁸ cm⁻³ or less(i.e., from about 2×10¹⁸ cm⁻³ to about 6×10¹⁸ cm⁻³), and its thicknessis about 0.3 μm or more and about 1.0 μm or less (i.e., from about 0.3μm to about 1.0 μm).

The high-concentration collector layer 31 is formed of n-type GaAs. TheSi concentration of the lower collector layer 31 a, a layer in thehigh-concentration collector layer 31, is about 1×10¹⁸ cm⁻³ or more andabout 5×10¹⁸ cm⁻³ or less (i.e., from about 1×10¹⁸ cm⁻³ to about 5×10¹⁸cm⁻³), typically about 3×10¹⁸ cm⁻³. The thickness of the lower collectorlayer 31 a is about 200 nm or more and about 900 nm or less (i.e., fromabout 200 nm to about 900 nm), typically about 500 nm. As can be seen,the lower collector layer 31 a has a dopant concentration similar to thedopant concentration of the subcollector layer 21 and a thicknesssimilar to the thickness of the subcollector layer 21. The middlecollector layer 31 b has a Si concentration of about 3×10¹⁶ cm⁻³ or moreand about 7×10¹⁶ cm⁻³ or less (i.e., from about 3×10¹⁶ cm⁻³ to about7×10¹⁶ cm⁻³), typically about 5×10¹⁶ cm⁻³, and a thickness of about 100nm or more and 300 nm or less (i.e., from about 100 nm to about 300 nm),typically about 200 nm. The upper collector layer 31 c has a Siconcentration of about 1×10¹⁶ cm⁻³ or more and about 4×10¹⁶ cm⁻³ or less(i.e., from about 1×10¹⁶ cm⁻³ to about 4×10¹⁶ cm⁻³), typically about1.5×10¹⁶ cm⁻³, and a thickness of about 100 nm or more and about 300 nmor less (i.e., from about 100 nm to about 300 nm), typically about 220nm.

The low-concentration collector layer 32 is formed of n-type AlGaAs, itsSi concentration is about 3×10¹⁵ cm⁻³ or less, typically about 3×10¹⁵cm⁻³, and its thickness is about 300 nm or more and about 500 nm or less(i.e. from about 300 nm to about 500 nm), typically about 400 nm.

The AlAs mixed-crystal ratio x in the graded collector layer 34 varieslinearly from about 0 to about 0.05 in the direction from the interfacewith the high-concentration collector layer 31 to the interface with thereversely graded collector layer 35. The thickness of the gradedcollector layer 34 is about 350 nm. The AlAs mixed-crystal ratio x inthe reversely graded collector layer 35 varies linearly from about 0.05to about 0 in the direction from the interface with the graded collectorlayer 34 to the interface with the base layer 40.

The base layer 40 is formed of p-type GaAs, its C concentration is about2×10¹⁹ cm⁻³ or more and about 5×10¹⁹ cm⁻³ or less (i.e., from about2×10¹⁹ cm⁻³ to about 5×10¹⁹ cm), and its thickness is about 50 nm ormore and about 150 nm or less (i.e., from about 50 nm to about 150 nm).

The emitter layer 41 is formed of n-type InGaP, its InP mixed-crystalratio is about 0.5, its Si concentration is about 2×10¹⁷ cm⁻³ or moreand about 5×10¹⁷ cm⁻³ or less (i.e., from about 2×10¹⁷ cm⁻³ to about5×10¹⁷ cm⁻³), and its thickness is about 30 nm or more and about 50 nmor less (i.e., from about 30 nm to about 50 nm).

The cap layer 42 is formed of n-type GaAs, its Si concentration is about2×10¹⁸ cm⁻³ or more and about 4×10¹⁸ cm⁻³ or less (i.e., from about2×10¹⁸ cm⁻³ to about 4×10¹⁸ cm⁻³), and its thickness is about 50 nm ormore and 150 nm or less (i.e., from about 50 nm to about 150 nm).

The contact layer 43 is formed of n-type InGaAs, and its Siconcentration is about 1×10¹⁹ cm⁻³ or more and about 3×10¹⁹ cm⁻³ or less(i.e., from about 1×10¹⁹ cm⁻³ to about 3×10¹⁹ cm⁻³. The thickness of thelower contact layer 43 a is about 30 nm or more and about 70 nm or less(i.e., from about 30 nm to about 70 nm), and the InAs mixed-crystalratio in this layer varies from about 0 to about 0.5 in the directionfrom the interface with the cap layer 42 to the interface with the uppercontact layer 43 b. The upper contact layer 43 b has an InAsmixed-crystal ratio of about 0.5 and a thickness of about 30 nm or moreand 70 nm or less (i.e., from about 30 nm to about 70 nm).

The collector electrode 45 has a multilayer structure in which, in orderfrom the bottom, an about 60-nm thick AuGe layer, an about 10-nm thickNi layer, an about 200-nm Au layer, an about 10-nm thick Mo layer, andan about 1-μm thick Au layer are stacked. The base electrode 46 andemitter electrode 47 have a multilayer structure in which, in order fromthe bottom, an about 50-nm thick Ti layer, an about 50-nm thick Ptlayer, and an about 200-nm thick Au layer are stacked.

The following describes a method for the production of an HBT accordingto Embodiment 2 with reference to FIG. 8A to FIG. 8D.

As illustrated in FIG. 8A, on a semi-insulating single-crystal GaAssubstrate 20, a subcollector layer 21, a high-concentration collectorlayer 31, a low-concentration collector layer 32, a base layer 40, anemitter layer 41, a cap layer 42, and a contact layer 43 are epitaxiallygrown one after another. This epitaxial growth can be performed using,for example, metalorganic chemical vapor deposition (MOCVD). The n-typedopant can be Si, and the p-type dopant can be C. The upper contactlayer 43 b may be doped with Se, Te, or any similar element to achieve ahigher concentration. The subcollector layer 21 may be doped with Te orany similar element to achieve a higher concentration.

As illustrated in FIG. 8B, an emitter electrode 47 is formed on theupper contact layer 43 b. Then, the upper contact layer 43 b, lowercontact layer 43 a, and cap layer 42 are etched away to the top surfaceof the emitter layer 41 using a photoresist mask in a predeterminedpattern as etching mask. After this etching, the photoresist mask, usedas etching mask, is removed. This leaves a mesa structure formed by theupper contact layer 43 b, lower contact layer 43 a, and cap layer 42.

As illustrated in FIG. 8C, a photoresist mask for patterning the emitterlayer 41, base layer 40, and collector layer 30 is formed. Using thisphotoresist mask as etching mask, the emitter layer 41, base layer 40,and collector layer 30 are etched away to the top surface of thesubcollector layer 21. After this etching, the photoresist mask, used asetching mask, is removed. This leaves a mesa structure formed by theemitter layer 41, base layer 40, and collector layer 30.

After that, the emitter layer 41 in the regions in which a baseelectrode 46 is to be formed is removed to expose the base layer 40. Onthe exposed base layer 40, a base electrode 46 is formed. After theformation of the base electrode 46, alloying is performed to achieve anohmic contact between the emitter electrode 47 and the upper contactlayer 43 b and an ohmic contact between the base electrode 46 and thebase layer 40.

As illustrated in FIG. 8D, a collector electrode 45 is formed on thesubcollector layer 21. After that, alloying is performed to achieve anohmic contact between the collector electrode 45 and the subcollectorlayer 21. Then, a protective film 49 is formed to cover the entire topsurface of the HBT. The protective film 49 can be, for example, asilicon nitride (SiN) film.

Although not particularly mentioned in the above description, it ispreferred to place any necessary etching stopper layer, a layer havingetching characteristics different from those of the semiconductor layersto be etched, at the interfaces at which etching should be stopped.

Advantages of Embodiment 2

The following describes great advantages of Embodiment 2.

In Embodiment 2, the AlAs mixed-crystal ratio x in the graded collectorlayer 34 (FIG. 7) at the point of the largest energy band gap (interfacebetween the graded collector layer 34 and the reversed graded collectorlayer 35) is set to about 0.05. That is, the maximum mixed-crystal ratiox_(MAX) is set to about 0.05. This mitigates, as can be seen from FIG.2, the blocking effect against electron transport that occurs in thecollector layer 30, and at the same time reduces the decrease in cutofffrequency that occurs in the presence of the Kirk effect. In otherwords, improvement in linear efficiency and limitation to the loss oflinear output power are achieved.

When the maximum mixed-crystal ratio x_(MAX) in and thickness of thegraded collector layer 34 (FIG. 7) are set to about 0.05 and about 400nm, respectively, the electron velocity is about 1.2×10⁷ cm/s, as shownin FIG. 6. This value is approximately 80% of the maximum electrondensity that can be achieved with an about 400-nm thick graded collectorlayer 34, about 1.5×10⁷ cm/s. A sufficiently high electron velocity istherefore attained. The maximum mixed-crystal ratio x_(MAX) may beselected from the range of about 0.03 or more and about 0.125 or less(i.e., about 0.03 to about 0.125). In this case, the electron velocityis equal to or faster than about 70% of the maximum. In particular,setting the maximum mixed-crystal ratio x_(MAX) to about 0.1 leads to anelectron velocity substantially equal to the maximum of about 1.5×10⁷cm/s.

It can be seen from FIG. 6 that when the thickness of the gradedcollector layer 34 is set to about 200 nm, the maximum mixed-crystalratio x_(MAX) can be selected from the range of about 0.025 or more andabout 0.125 or less (i.e., from about 0.025 to about 0.125). Inparticular, setting the maximum mixed-crystal ratio x_(MAX) to about0.05 leads to an electron velocity substantially equal to the maximumelectron velocity of about 1.8×10⁷ cm/s.

When the thickness of the graded collector layer 34 falls within therange of about 200 nm or more and about 600 nm or less (i.e., from about200 nm to about 600 nm), setting the maximum mixed-crystal ratio x_(MAX)to about 0.025 or more and about 0.125 or less (i.e., from about 0.025to about 0.125) leads to an electron velocity approximately 70% of themaximum electron velocity. This results in a further increase in theoutput power and efficiency of the HBT.

In Embodiment 2, a lower collector layer 31 a having a dopantconcentration substantially equal to the dopant concentration of thesubcollector layer 21 is interposed between the middle collector layer31 b and the subcollector layer 21. The lower collector layer 31 areduces the collector resistance by acting as resistor inserted inparallel with the subcollector layer 21. This results in a furtherincrease in the output power and efficiency of the HBT.

In Embodiment 2, in the formation of the collector layer 30 in theepitaxial growth step illustrated in FIG. 8A, the graded collector layer34 and reversely graded collector layer 35 can be formed sequentially onthe high-concentration collector layer 31. Moreover, in the etching stepillustrated in FIG. 8B, it is possible to remove the contact layer 43and cap layer 42 sequentially. In the etching step illustrated in FIG.8C, continuous etching from the emitter layer 41 to the surface of thesubcollector layer 21 is possible. Thus, there is no need to add a newstep to a known process for the production of an HBT, in which thegraded collector layer 34 and reversely graded collector layer 35 arenot provided. In this way, the manufacturer can produce an HBT accordingto Embodiment 2 using a known process for the production of an HBT as itis, without causing the complication of the process, and, therefore, canavoid production cost increases.

In Embodiment 2, the dopant concentrations of the middle collector layer31 b, upper collector layer 31 c, and low-concentration collector layer32 are equal to or less than about 1/10 of the dopant concentrations ofthe subcollector layer 21 and lower collector layer 31 a. Setting dopantconcentrations as such prevents a decrease in base-collector breakdownvoltage and collector emitter breakdown voltage. As a result, thebreakdown of the HBT is prevented that would otherwise occur when theoutput voltage swings to a maximum extent at full high-frequency outputpower.

In Embodiment 2, the low-concentration collector layer 32 is n-type, andits dopant concentration is set to about 3×10¹⁵ cm⁻³ or less. Settingthe concentration of the low-concentration collector layer 32 lowerfurther improves the linear efficiency. In other configurations, thelow-concentration collector layer 32 may be formed of p-type AlGaAs witha C concentration of about 1×10¹⁵ cm⁻³ or less, or alternatively thelow-concentration collector layer 32 may be formed of intrinsic AlGaAs.

Variation of Embodiment 2

Although in Embodiment 2 the low-concentration collector layer 32 isformed into a two-layer structure composed of a graded collector layer34 and a reversely graded collector layer 35, the reverse gradedcollector layer 35 is optional.

FIG. 9 is a schematic cross-sectional view of an HBT according to avariation in which the reversely graded collector layer 35 is notprovided. The entire low-concentration collector layer 32 is a gradedcollector layer 34. The energy band gap of the graded collector layer 34is the largest at the interface between the graded collector layer 34and the base layer 40. Even such a configuration, in which the reverselygraded collector layer 35 is not provided, gives advantages similar tothose in Embodiment 2.

Although in Embodiment 2 the high-concentration collector layer 31 (FIG.7) is structured into three layers with different dopant concentrations,this layer may be formed into a single-layer or two-layer structure, oralternatively into a multilayer structure composed of four or morelayers. Moreover, the two layers of the middle collector layer 31 b andupper collector layer 31 c may be replaced with one layer in which thedopant concentration is varied so that the dopant concentration becomeshigher with increasing distance from the interface with thelow-concentration collector layer 32. It is also possible to replace thethree layers constituting the high-concentration collector layer 31 withone layer in which the dopant concentration is varied so that the dopantconcentration becomes higher with increasing distance from the interfacewith the low-concentration collector layer 32. A more commonconfiguration can also be used in which the high-concentration collectorlayer 31 is structured into a single layer or multiple layers at leastone of which is configured such that the dopant concentration becomesgradually higher from the base layer 40 side to the subcollector layer21 side.

Other Variations

Although in Embodiments 1 and 2 the emitter layer, base layer, andcollector layer are InGaP, GaAs, and AlGaAs layers, respectively, thetechnical ideas behind the HBTs according to Embodiments 1 and 2 canalso be applied to HBTs such as HBTs of InGaAsP/GaAs type, HBTs ofAlGaAs/GaAs type, HBTs of InGaP/GaAsSb type, HBTs of InP/InGaAs type,HBTs of InAlAs/InGaAs type, HBTs of Si/SiGe type, HBTs of AlGaN/GaNtype, and HBTs of GaN/InGaN type. For example, the emitter layer/baselayer/collector layer combination can be selected from combinations suchas InGaP/GaAs/GaInNAs, AlGaAs/GaAs/AlGaAs, AlGaAs/GaAs/GaInNAs,InGaP/InGaAs/AlGaAs, InGaP/InGaAs/GaInNAs, InGaP/GaAsSb/AlGaAs,InGaP/GaAsSb/GaInNAs, InGaP/AlGaAs/AlGaAs, InGaP/AlGaAs/GaInNAs,InGaP/GaInNAs/AlGaAs, and InGaP/GaInNAs/GaInNAs.

In Embodiments 1 and 2, the graded collector layer 34 is included in thelow-concentration collector layer 32. A graded semiconductor layer inwhich the energy band gap narrows from the base layer 40 side to thesubstrate 20 side may be provided extending from the low-concentrationcollector layer 32 to part of the high-concentration collector layer 31.That is, part of the high-concentration collector layer 31 may be agraded semiconductor layer that has energy band gaps narrowing from thelow-concentration collector layer 32 side to the substrate 20 side.

While preferred embodiments of the disclosure have been described above,it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the disclosure. The scope of the disclosure, therefore, isto be determined solely by the following claims.

What is claimed is:
 1. A heterojunction bipolar transistor comprising asubstrate; and a multilayer structure on the substrate, the multilayerstructure including a collector layer, a p-type base layer, and ann-type emitter layer, wherein the collector layer includes ahigh-concentration collector layer and a low-concentration collectorlayer between the base layer and the high-concentration collector layer,the low-concentration collector layer having a lower dopantconcentration than the high-concentration collector layer does, thelow-concentration collector layer includes a graded collector layer, andthe graded collector layer includes AlGaAs, an AlAs mixed-crystal ratiodecreases with increasing distance from the base layer, and the AlAsmixed-crystal ratio in the graded collector layer at an interface on abase layer side is equal to or less than about 0.125.
 2. Theheterojunction bipolar transistor according to claim 1, wherein the AlAsmixed-crystal ratio in the graded collector layer at the interface onthe base layer side is equal to or more than about 0.025.
 3. Theheterojunction bipolar transistor according to claim 1, wherein the baselayer includes GaAs, GaAsSb, InGaAs, or InGaAsN.
 4. The heterojunctionbipolar transistor according to claim 1, further comprising an n-typesubcollector layer on the substrate, wherein the collector layer, baselayer, and emitter layer are stacked in this order on part of a surfaceregion of the subcollector layer, and the high-concentration collectorlayer includes a first layer on a subcollector layer side and a secondlayer on a low-concentration collector layer side, and a dopantconcentration of the first layer and a dopant concentration of thesubcollector layer are higher than a dopant concentration of the secondlayer.
 5. The heterojunction bipolar transistor according to claim 1,wherein the low-concentration collector layer includes a reverselygraded collector layer between the graded collector layer and the baselayer, the reversely graded collector layer includes AlGaAs, and an AlAsmixed-crystal ratio varies in a thickness direction, at an interfacebetween the graded collector layer and the reversely graded collectorlayer, the AlAs mixed-crystal ratio in the graded collector layer andthe AlAs mixed-crystal ratio in the reversely graded collector layer areequal, and the AlAs mixed-crystal ratio in the reversely gradedcollector layer at an interface on a base layer side is less than theAlAs mixed-crystal ratio in the reversely graded collector layer at theinterface between the graded collector layer and the reversely gradedcollector layer.
 6. The heterojunction bipolar transistor according toclaim 1, where a dopant concentration of the high-concentrationcollector layer has a tendency to increase from a low-concentrationcollector layer side to an opposite side of the low-concentrationcollector layer.
 7. The heterojunction bipolar transistor according toclaim 4, wherein the dopant concentration of the second layer and adopant concentration of the low-concentration collector layer are equalto or less than about 1/10 of the dopant concentration of the firstlayer.
 8. The heterojunction bipolar transistor according to claim 6,wherein the dopant concentration of the high-concentration collectorlayer at the low-concentration collector layer side and a dopantconcentration of the low-concentration collector layer are equal to orless than about 1/10 of a dopant concentration of the high-concentrationcollector layer at an opposite side of the low-concentration collectorlayer.
 9. A heterojunction bipolar transistor comprising a substrate;and a multilayer structure on the substrate, the multilayer structureincluding a collector layer, a p-type base layer, and an n-type emitterlayer, wherein the collector layer includes a high-concentrationcollector layer and a low-concentration collector layer between the baselayer and the high-concentration collector layer, the low-concentrationcollector layer having a lower dopant concentration than thehigh-concentration collector layer does, and the low-concentrationcollector layer includes a graded collector layer that includes InGaAsN.10. The heterojunction bipolar transistor according to claim 9, whereinan energy band gap of the graded collector varies to narrow withincreasing distance from the base layer.
 11. The heterojunction bipolartransistor according to claim 9, the base layer includes GaAs, GaAsSb,InGaAs, or InGaAsN.
 12. The heterojunction bipolar transistor accordingto claim 9, further comprising an n-type subcollector layer on thesubstrate, wherein the collector layer, base layer, and emitter layerare stacked in this order on part of a surface region of thesubcollector layer, and the high-concentration collector layer includesa first layer on a subcollector layer side and a second layer on alow-concentration collector layer side, and a dopant concentration ofthe first layer and a dopant concentration of the subcollector layer arehigher than a dopant concentration of the second layer.
 13. Theheterojunction bipolar transistor according to claim 9, wherein thelow-concentration collector layer includes a reversely graded collectorlayer between the graded collector layer and the base layer, an energyband gap of the reversely graded collector layer varies in a thicknessdirection, and at an interface between the graded collector layer andthe reversely graded collector layer, an energy band gap of the gradedcollector layer and the energy band gap of the reversely gradedcollector layer are equal, and the energy band gap of the reverselygraded collector layer at an interface on a base layer side is less thanthe energy band gap of the reversely graded collector layer at theinterface between the graded collector layer and the reversely gradedcollector layer.
 14. The heterojunction bipolar transistor according toclaim 9, where a dopant concentration of the high-concentrationcollector layer has a tendency to increase from a low-concentrationcollector layer side to an opposite side of the low-concentrationcollector layer.
 15. The heterojunction bipolar transistor according toclaim 12, wherein the dopant concentration of the second layer and adopant concentration of the low-concentration collector layer are equalto or less than about 1/10 of the dopant concentration of the firstlayer.
 16. The heterojunction bipolar transistor according to claim 13,the energy band gap of the reversely graded collector layer at theinterface on the base layer side is equal to an energy band gap of thebase layer.
 17. The heterojunction bipolar transistor according to claim14, wherein the dopant concentration of the high-concentration collectorlayer at the low-concentration collector layer side and a dopantconcentration of the low-concentration collector layer are equal to orless than about 1/10 of the dopant concentration of thehigh-concentration collector layer at an opposite side of thelow-concentration collector layer.